Reductive carbon dots for reduction, ratiometric fluorescence determination, and intracellular imaging of Au³

Many studies show that ortho-phenylenediamine (OPD) produces an oxidized fluorescent product when exposed to an oxidizing agent that enables the direct or indirect fluorescence detection of a range chemical and biochemical analytes. However, there is no report on this unique optical behavior for other two isomers of phenylenediamine. This study demonstrates that a simple hydrothermal treatment of para-phenylenediamine (PPD) in the presence of sulfuric acid results in the formation of fluorescent N, S-doped carbon dots (CDs) with triple functionalities including the reduction of Au³⁺ into gold nanoparticles (AuNPs), the stabilization of the produced AuNPs, and the determination of Au³⁺ concentration through an intrinsic ratiometric fluorescence signal. In the presence of Au³⁺, the blue emission of CDs at 437 nm quenched, and a green emission at 540 nm emerged. The linear concentration range for the determination of Au³⁺ was 20 nM-16 µM with a detection limit of 16 nM. Additionally, the dual emissive CDs-AuNPs hybrid probe showed potential for the indirect fluorescence ratiometric determination of cysteine and sulfide ions. The linear concentration range for cysteine and sulfide ions were 0.25-8 μM and 0.1-6 μΜ, with detection limits of 0.095 μM and 0.041 μM, respectively. Accordingly, CDs were applied to detect Au³⁺ and S^2- in real water samples. Moreover, the synthesized CDs showed no cytotoxicity for HeLa cells up to 300 µg mL^-1, as determined by the MTT assay. Therefore, their potential for intracellular imaging of Au³⁺ in living cells was also investigated.